如何将UILabel放在UIToolbar的顶部？

// 创建UIToolbar对象
let toolbar = UIToolbar(frame: CGRect(x: 0, y: 0, width: view.frame.width, height: 44))
view.addSubview(toolbar)

// 创建UILabel对象
let label = UILabel()
label.text = "Hello World"
label.font = UIFont.systemFont(ofSize: 17)
label.textColor = UIColor.black
label.sizeToFit()

// 创建UIBarButtonItem对象
let labelItem = UIBarButtonItem(customView: label)

// 将UIBarButtonItem添加到UIToolbar的items数组中
toolbar.items = [labelItem]